If a tree falls in Maryland's forests — even if no one hears it — researchers soon will have a handle on how much it could contribute to global warming.

A pair of geographical scientists at the University of Maryland, College Park is leading an ambitious effort to map the state's forests and measure changes over time in the amount of carbon stockpiled in the trees. With a $1.4 million grant from NASA, the research team hopes to use satellite imagery, aerial photography and ground observations to develop new methods for tracking the carbon stored in woodlands, which could be applied locally, nationally and globally.

"We would like to know for Maryland what is the current carbon that is being stored in [the state's] forests and how much can we expect them to continue to pull out of the atmosphere as we go forward," said Professor Ralph Dubayah, principal investigator on the project.

Trees are commonly referred to as carbon "sinks," meaning that they consume carbon dioxide from the air and lock up the climate-altering element in their wood. Forests release some carbon dioxide naturally as they burn energy in growth and also as trees die and decay. But as long as a forest is healthy and growing, it absorbs more carbon than it releases.

While much attention has been focused on tallying emissions of carbon dioxide and other greenhouse gases from power plants, vehicles and buildings, there's less understanding of the role forests play in preventing or causing warming, and the impact of land uses such as logging, suburbanization and tree-planting.

"The biggest uncertainty we have in our carbon budget — about how much carbon dioxide is going to wind up in our atmosphere in 10, 20 or 30 years — is not from [the use of] fossil fuels," Dubayah said. "It's what the net effect will be from deforestation and subsequent regrowth."

NASA has been directed by Congress to develop ways of monitoring forest carbon using satellites and other resources. The grant to the UM-led team aims to do that for the entire state, building on a pilot project that mapped the forest "biomass" or carbon content in two Maryland counties, Anne Arundel and Howard.

Internationally, developing countries are being encouraged or even paid to preserve or replant their forests as a means of combating climate change from emissions elsewhere. But Dubayah said the REDD program (for Reducing Emissions from Deforestation and Degradation) lacks the means to reliably track forest activity on such a large scale.

In this country, some states, including Maryland, are committed to reducing their greenhouse gas emissions, but California is the first to charge every industry for its climate-warming releases. The state allows targeted industries to offset their emissions by planting trees, among other things, but the only way to verify that the trees are still there and growing is to send people out to measure tree trunks with tape measures every year or so, a cumbersome and costly enterprise.

Working with the U.S. Forest Service, the University of Vermont and a pair of private aerial sensing firms, the UM researchers are integrating satellite imagery and LIDAR airborne laser measurements of the state's forests and woodlots. In addition to laying out the wooded areas' location and extent, the group also is tallying the heights of trees in order to calculate their biomass.

Researchers are ground-truthing and resolving inconsistencies in their remotely collected data by having a pair of forest service workers go out and check randomly selected spots in Maryland.

Last summer and fall, research foresters Katie Kranich and David Bruhn crisscrossed the state to see and measure what trees could be found on a variety of spots, from the forested mountains of Western Maryland to the marshes of the Eastern Shore. Around Baltimore, they knocked on doors occasionally to seek permission to count and measure urban and suburban trees, using a tape measure to get their girth and a hand-held gadget known as a hypsometer to calculate the height of the tree.

"Some people are definitely wary of having strangers on their property," said Kranich, 24, a Philadelphia-area native who graduated from Drexel University with a degree in environmental science, "which is totally understandable."

Their survey included nonforested plots as well as forested ones, so they occasionally found themselves sent to a spot with few trees. Woods bordered a White Marsh apartment complex they visited last month, but there were only a handful of oak, red maple, beech and tupelo trees in the patch of ground to which they were directed in the middle of the parking lot. In another case, the GPS location given them to check took them to a lone redbud in the middle of Interstate 68 in Western Maryland.

"They're just trying to find trees that haven't been accounted for before," said Bruhn, 24, who grew up in New Jersey and graduated from Michigan State University, where he complemented his liberal arts major with a student conservation internship.

Dubayah said sampling on the ground helps verify measurements taken from aircraft, so researchers can analyze imagery and data for large areas with confidence, eventually for bigger states, nationwide and globally. But the technique also could be focused in on a neighborhood or on individual yards, to help a property owner calculate his or her "carbon footprint."

"You would know about how much carbon your backyard has," the UM researcher said. "You would know the implications of, 'If I cut this down, how much carbon would I lose?'"